A conventional type of alkaline cell employs a cathode comprising predominantly an oxidic depolarizer such as manganese dioxide usually admixed with a binder and conductive material such as graphite, steel wool and the like. The anode usually comprises a consumable anodic material such as powder zinc admixed with a gelling agent such as carboxymethyl cellulose, a suitable alkaline electrolyte such as an aqueous potassium hydroxide solution and, if desired, mercury. The anode gel is then extruded to form a desired shaped electrode. Anode materials and their preparation are described in U.S. Pat. Nos. 2,938,064 and 2,935,547 issued to K. Kordesch and U.S. Pat. No. 2,993,947 issued to E. E. Leger. The above electrodes together with conventional separator material can be assembled into an alkaline-MnO.sub.2 /zinc cell.
Discharging the above described cell on a microampere drain or on very intermittent discharge has resulted in zinc oxide formation in the separator which can result in internal shorts. Specifically, zinc oxide in contact with metallic zinc is known to be conductive and an internal electronic path is formed between the anode and cathode. An investigation of the internal shorting problem has revealed the carboxymethyl cellulose to be a major contributor to the transporting and precipitating of the zinc oxide throughout the cell. This internal shorting causes wasteful consumption of the components of the cell resulting in lower output capacity.
In eliminating or reducing carboxymethyl cellulose in powder-gel anodes, U.S. application Ser. No. 892,313 now U.S. Pat. No. 4,209,577 discloses a powder-gel anode comprising a major portion of a consumable anode material such as zinc, a minor amount of a gelling agent such as methyl cellulose with or without carboxymethyl cellulose and an aqueous alkaline electrolyte such as an aqueous potassium hydroxide solution. U.S. Pat. No. 4,175,052 discloses a powder-gel anode comprising a major portion of a consumable anodic material such as zinc, a minor amount of a gelling agent such as poly-N-vinyl pyrrolidone and/or polymethacrylic acid with or without carboxymethyl cellulose and an aqueous alkaline electrolyte such as an aqueous potassium hydroxide solution. Both the '313 application and the '052 patent are incorporated herein by reference.
Although a gelling agent is required, it is desirable to utilize a gelling agent that enables a cell to function at a given efficiency with a minimum amount of the gelling agent so that a larger volume of the active components such as zinc and manganese dioxide can be put in the cell. The gelling agent is particularly important for cells intended to operate on high discharge rates such as is required for toys, movie cameras, camera flash units and the like. The efficiency of anode utilization (normally the limiting factor in high rate service) is strongly affected by the gelling agent since the gelling agent affects the distribution and availability of electrolyte and the distribution of discharge products. In addition, it functions to keep the particulate anode material in a suspended state for greater electrolyte availability and better current density distribution. Consequently, the selection of a gelling agent is of primary importance in the fabrication of powder-gel anodes.
It is an object of the present invention to provide an alkaline-MnO.sub.2 /zinc cell that can be discharged on microampere drains without internally shorting and also on high current drains at effectively high anode utilization.
It is another object of the present invention to provide a powder-gel anode for alkaline-MnO.sub.2 cells that requires less of a gelling agent thereby permitting more of the active anodic material to be employed in a given anode mix volume.
It is another object of the present invention to provide a powder-gel anode for alkaline-MnO.sub.2 cells that employs a starch graft copolymer with or without another gelling agent.
It is another object of the present invention to provide a powder-gel anode comprising powder zinc, a starch graft copolymer with or without another gelling agent and an aqueous alkaline electrolyte.
The foregoing and additional objects will become more fully apparent from the following description.